Alzheimer's disease (AD) is a neurodegenerative disease characterized by the accumulation of amyloid plaques in the brain. These plaques are composed of mostly A? peptides generated by proteolysis of the amyloid precursor protein (APP) by two proteases, ?- and ?-secretase. The primary cleavage product is an A? peptide with a length of 40 residues (A?40). However, proteolysis is not highly specific and ~10% of the cleavage products of APP are peptides with two additional amino acids (A?42). The A?42 peptide is more toxic than A?40, and is the principal component of amyloid plaques in the brain. The overarching goal of the proposed research is to establish the mechanism of inhibition for small molecule inhibitors that target neurotoxic A? oligomers in order to design more effective inhibitors. The approach is to combine structural methods with functional assays to determine A?42 structure-function relationships in three specific aims. The first aim is to determine the structure and toxicity of the soluble oligomers and fibrils of A?42 using a suite of methods including solution and solid-state nuclear magnetic resonance (NMR) spectroscopy, single touch atomic force microscopy and Fourier transform infrared (FTIR) spectroscopy. The second aim is to determine the structure of membrane-bound oligomers and the dynamics of oligomer- membrane interactions. Single molecule total internal reflection fluorescence microscopy will be used to establish the association-dissociation rates and distribution of A?42 bound to membrane bilayers. FTIR spectroscopy will be used to characterize the changes in secondary structure as a function of membrane composition. Solution-state NMR and solid-state NMR spectroscopy will be used to follow specific structural markers identified in Aim 1 that are unique to the oligomers, protofibrils and fibrils. The third aim is to determine the mechanism of interaction of small molecule, peptide and protein inhibitors with A? oligomers and fibrils. The small molecule inhibitors include the natural products, curcumin and resveratrol. The peptide inhibitors are designed on the basis of the structure of the A? fibrils. The protein inhibitors are derived from fragments of the myelin basic protein, which we have shown is a natural A? inhibitor in brain white matter. An improved understanding of A?-inhibitor interactions will impact the design of inhibitors to the soluble oligomers. The goal is to establish 1) how the neurotoxic soluble oligomers differ in structure from membrane-bound oligomers and A?42 fibrils, 2) how the addition of two amino acids changes the structure of the A?42 oligomers and fibrils compared to the less toxic A?40 form, and 3) how inhibitors bind to A?42 and prevent toxicity.